FIG. 1 illustrates a prior-art communication 100 system having an integrated-circuit receiver/transmitter 101 and a frequency-selective input or output filter 103. The filter 103 is useful for suppressing out-of-band spurious signals, reducing dynamic range, eliminating harmonic distortions, power/noise matching to antennas, and so forth. In the case of a television receiver, for example, the filter 103 may be disposed at the signal input source (antenna 107) to eliminate interference from noise sources (e.g., cellular telephones and other radio-frequency (RF) devices) and to minimize the amount of extraneous power processed by subsequent receiver stages within IC 101. Furthermore, in the case of a television receiver, the filter 103 can be used to power match the receiver antenna, improving its reception performance and sensitivity.
Frequency-selective filters typically need to be tuned to center their passband within the frequency band of interest. For systems that have an intrinsically wide tuning range (either across a single wide-frequency band or across multiple smaller bands), such tuning can be extremely challenging to implement, often requiring passive inductors, capacitors, varactors and other components that further exacerbate the tuning challenge. In the system of FIG. 1, for example, a varactor (V1) is provided to enable the resonant frequency of a tank circuit formed by the varactor and an inductor (L1) to be adjusted through application of a 0-30 volt varactor bias voltage (the capacitance of a varactor is generally proportional to the inverse square-root of the bias voltage so that a 30 volt bias range enables a roughly 5-6× adjustment of varactor capacitance). In a typical implementation, a dedicated phase-locked-loop (PLL) device 105 fabricated in a 30-volt analog IC process is used to develop the varactor bias voltage. More specifically, a varactor is commonly provided as the charge storage device within the PLL device 105 (i.e., storing the output of a charge pump and thus developing the control voltage used to determine the oscillation rate of a voltage-controlled-oscillator (VCO)) so that, as a channel-select signal 110 is switched to select a passband of interest (e.g., by selecting a ratio between the frequency of the VCO output and the frequency of a reference clock signal), the voltage developed on the PLL varactor is increased or decreased and thus may be output to the filter 103 as the varactor bias voltage. Typically, inductor L1 is adjusted manually to calibrate the filter passband for a given channel selection, thus slaving varactor V1 to the PLL varactor so that, as the channel-select signal is changed to select different channels within the broader frequency band, the varactor bias voltage is adjusted accordingly to establish the desired passband within the filter.
A major disadvantage of the above-described filter-tuning arrangement, aside from the added cost of the dedicated PLL IC, is that tuner calibration (i.e., the slaving of varactor V1 to the PLL varactor) is generally performed only once, at system production time, and thus fails to account for run-time temperature and voltage variations. System 100 is also susceptible to loss of calibration due to component aging, or from vibration, shock or other physical perturbations common in mobile applications, which tends to disturb the setting of the manually adjusted inductor.